In recent years, for the purpose of ensuring safety of vehicle occupants at the time of a vehicle crash and improving fuel economy by reducing the weight of a vehicle body, a high strength cold rolled steel sheet having a tensile strength of 750 MPa or more and a small thickness is actively used as a structural member of a vehicle. In order to manufacture such a steel sheet, it is effective to use a continuous annealing line equipped with a water quenching device so as to increase the volume ratio of a martensitic phase in the steel sheet (refer to Patent Literature 1). That is, a steel sheet is heated until reaching a temperature (water quenching temperature) at which the structure of the steel sheet becomes a mixed structure of a ferrite phase and an austenitic phase or a structure of an austenitic single phase, and thereafter, the steel sheet is immersed into water in the water quenching device so as to be cooled at a critical cooling rate or higher. Thus, it is possible to manufacture a steel sheet having a mixed structure of a ferrite phase and a martensitic phase or a structure of a martensitic single phase. The volume ratio of the martensitic phase increases as the water quenching temperature becomes higher, and the strength of the steel sheet becomes higher in proportion to the increase in the volume ratio of the martensitic phase.
When such water quenching treatment as described above is applied to increase the strength of a steel sheet, the steel sheet may be curved in a circular arc in the sheet width direction, and thus may no longer be flat after the water quenching treatment although having been flat before the water quenching treatment. This is because a rapid thermal contraction occurs due to a rapid temperature drop by the water quenching treatment, and the thermal contraction causes the steel sheet to buckle. If the flatness of the steel sheet is worsened, the threading performance in the continuous annealing line drops, thereby causing a drop in a feed speed of the steel sheet or a threading trouble, thus posing problems in the next process such as a stamping process. From such a background, there is proposed a method for suppressing the circular arc-shaped curvature associated with the water quenching treatment. Specifically, Patent Literature 2 discloses a technology that straightens the deformed portion of the steel sheet into a flat shape by applying a pressure obtained by pressing, during the water quenching treatment, the entire area in the width direction of front and rear surfaces of the steel sheet.